1. Field of the Invention
The present invention relates to a light detecting device and a method of controlling a light detecting device.
2. Description of the Related Art
Conventionally, a photo transistor and a charge-coupled device/complementary metal-oxide semiconductor (CCD/CMOS) image sensor are used as a light detecting device. The light detecting device may be used, for example, as an image sensor in a video camera and a digital camera for reading out the intensity of a detected light as a change in voltage. The light detecting device may also be used as an optical communication apparatus using light as transmission media. The light detecting device may also used as a sensor in an image data generating apparatus that generates range image data by arranging pixel values in accordance with a distance to a detected object using an active light.
The range image is an image in which a distance to a detected object is expressed with light and shade (pixel values). By using the range image, distances to respective objects within a large area can be detected at one time.
When the light detecting device detects a signal light caused by a light source, that is, when the light detecting device is used, for example, for a perimeter monitoring sensor, an infrared ray communication, or a distance sensor for a robot vision, an outside light enters in addition to the signal light caused by the light source. In the above-described case, the intensity of detected light is increases by the intensity of the outside light. Therefore, it is difficult to detect the intensity of the signal light with accuracy.
In the light detecting device, charges corresponding to the amount of the detected light are generated and are stored. However, there are limitations to the amount of changes that can be stored, and the stored charges may be saturated with increase in the amount of the detected light. Thus, when a high-intensity outside light enters in addition to the signal light, the light detecting device may be saturated only by the outside light, and the signal light may not be read out.
Furthermore, in a case where the intensity of the outside light fluctuates with time, the intensity of the detected light also fluctuates. As a result, a component corresponding to the signal light and a component corresponding to the outside light cannot be separated. A method for separating a component corresponding to a signal light and a component corresponding to an outside light from a light detection signal of a light detecting device is disclosed, for example, in US 2006/0192938 A1 (corresponding to JP-A-2004-294420) and US 2007/0103748 A1 (corresponding to JP-A-2005-303268).
In the method disclosed in US 2006/0192938 A1, a light source is intermittently turned on, difference between a light detection signal during a light-on time and a light detection signal during a light-off time is calculated, and thereby a component corresponding to the outside light is removed. In other words, in a short time in which the intensity of the outside light does not change, the light detection signal during the light-off time including only the component corresponding to the outside light is subtracted from the light detection signal during the light-on time including a component corresponding to the light signal and the component corresponding to the outside light. As a result, the ratio of the component corresponding to the outside light can be increased by reducing the ratio of the component corresponding to the outside light.
In the invention disclosed in US 2006/0192938 A1, at least two charge storing nodes are required for one light detecting device in order to detect difference in phases of the signal light and the light source. Thus, the dimension of a process circuit in a pixel circuit increases.
In addition, another charge storing node is also required in order to store the charges due to the outside light, separate the component corresponding to the signal light and the component corresponding to the outside light, and remove the component corresponding to the outside light. Thus, the dimension of the process circuit in the pixel circuit further increases. As a result, the number of transistors included in one pixel increases, and the light detecting signal is difficult to be used for a high-pixel device.
Furthermore, in order to remove the influence of the outside light, at least two output operations, that is, an output operation of the light detection signal during the light-on time including the component corresponding to the signal light and the component corresponding to the outside light and an output operation of the light detection signal during the light-off time including only the component corresponding to the outside light. Thus, a response time required to detect the light detection signal corresponding to the signal light increases.
In a case where a weak signal such as a reflected signal from a long distance or a reflected signal from a black object or an object having a low reflectivity is treated, an integral time of the light detecting device may be increased in order to increase the amount of detected light. In this case, the response time required to detecting the light detection signal corresponding to the signal light further increases. Furthermore, a time-lag of the light detection signal of the component corresponding to the outside light may increase, and the intensity of the outside light may fluctuate. As a result, it may be difficult to remove the component corresponding to the outside light with accuracy.
In order to the remove the component corresponding to the outside light efficiently, each of the charge storing nodes must not be saturated. The detected charges are transferred to each of the charge storing nodes and the light detection signal at the time is read out as a voltage value. When a high-intensity light enters, the charge storing nodes may be saturated after the charges are transferred, and a light detection signal having incorrect voltage may be read out. As a result, in the outside light greater than the saturation limit of each of the charge storing nodes, it is difficult to detect the component corresponding to the signal light.
In the method disclosed in US 2007/0103748 A1, electrons and holes are separately stored in a light detecting device as target carriers and non-target carriers. Then, a component corresponding to an outside light is removed by selectivity recombining difference between a light-on time and a light-off time. In the method, charges can be cancelled as pairs of electron and hole, and the component corresponding to the outside light can be removed before reading out of the light detecting device. Thus, a component corresponding to the signal light can be detected while restricting saturation due to the outside light, and a dynamic range for light can be improved.
In the light detecting device, various operations such as holding, discharging, and recombining of electrons and holes can be performed by controlling control electrodes in a light detecting part in order to restrict saturation of charge storing nodes.
During the light-off time of the light source, charges due to the outside light are divided into electrons and holes, and the electrons and the holes are separately held. Next, only the electrons are discharged. During the light-on time of the light source, charges due to the signal light and the outside light are divided into electrons and holes, and the electrons and the holes are separately held. Then, the electrons and the holes are recombined, and the remaining charges are transferred as a signal light component to the charge storing node. Thus, the light detecting device can output the signal light component in which an outside light component is not included.
In order to use the remaining charges after recombination as the signal light component, the light detecting device needs to be configured so that a ratio of electrons and holes generated by light to be 2:1. The ratio of the electrons and the holes depends on a structure of an element (for example, a positional relationship, shapes, dimensions, and impurity concentrations of an electron holding region and a hole holding region) and a difference in motilities of the electrons and the holes. Thus, the ratio of the electrons and the holes depends on an accuracy of a manufacturing process.
A height of a potential barrier and a depth of a potential well depend on an arrangement of the control electrodes for controlling the electrons and the holes and a voltage applied to the control electrodes. The ratio of the electrons and the holes generated by a light changes with the height of the potential barrier and the depth of the potential well. Thus, it is difficult to actually control the ratio of the electrons and the holes to be 2:1 in the light detecting device.
Furthermore, since the light detecting device has a complicated structure, the light detecting device cannot be formed without a special process. Thus, the manufacturing process may become complicated, development costs and develop time may increase, and a competitiveness may be reduced. In addition, since it is difficult for a manufacturer without own process to tune up the light detecting device, a growth of a market is restricted.